nir/loop_unroll: Prepare loop for unrolling in wrapper_unroll

Without loop_prepare_for_unroll loops are losing phis.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111411


Fixes: 5db98195 "nir: add loop unroll support for wrapper loops"
Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
(cherry picked from commit 84b3ef6a)